The present invention relates generally to airflow sensors for automotive vehicles. More particularly, the invention relates to an airflow sensor which uses a microcomputer-based digital control circuit for improved accuracy, better performance and easier calibration.
With today's emphasis on pollution control, economy and engine efficiency, vehicle engines are becoming quite sophisticated. Today's vehicles employ one or more onboard computer systems with associated sensors for monitoring vital engine operating parameters, in order to minimize pollution and to maximize economy and engine efficiency. One such vital parameter is airflow. Airflow has a direct bearing on engine performance and also indirectly indicates engine loading.
Airflow sensors are conventionally used to provide the vehicle onboard computer with a signal indicative of the mass airflow at a given point in time. Traditionally, the mass airflow sensor has been a major source of error, giving rise to both poor performance, poor efficiency, and unacceptable pollution control. The conventional hot wire mass airflow sensor operates on the wind chill principle. The conventional hot wire sensor has a thermal foil element positioned within a flow confining venturi conduit connected to the air intake system of the vehicle. The thermal foil element is connected to a Wheatstone bridge circuit which receives electrical energy from a power source to heat the thermal foil element to a predetermined temperature. As the mass airflow across the thermal foil element increases, the temperature of the element drops causing a corresponding decrease in the resistance of the thermal element. This decrease unbalances the bridge circuit. By adding electrical energy to the bridge circuit, the temperature of the thermal foil element can be brought back to the original predetermined temperature, whereupon the resistance of the element returns to its original value and the bridge circuit is once again balanced. Thus the level of electrical energy applied to the supply node of the bridge circuit gives an indication of the mass airflow across the thermal foil element.
While workable in theory, the above-described conventional mass airflow sensor is difficult to calibrate and expensive to mass produce. Also, as the device responds to temperature change in the thermal foil element, it is also susceptible to inaccuracies due to changes in the ambient air temperature. While thermistor circuits have been used to take ambient air temperature into account, these circuits do not fully correct the problem, since the ambient air temperature can affect in a nonlinear fashion the sensor's response to changes in mass airflow.
The control circuit for a conventional mass airflow sensor is an analog circuit comprising a bridge control circuit for maintaining the bridge in a balanced condition by regulating the quantity of electrical energy applied to the supply node of the bridge. The conventional analog circuit also comprises temperature compensation amplifiers and calibration amplifiers which, to a limited degree, compensate for inaccuracies of the bridge circuit to provide an analog voltage indicative of the mass airflow. This analog voltage is fed to a voltage controlled oscillator which produces a variable frequency signal indicative of the mass airflow.
In order to initially calibrate the conventional sensor, the resistors comprising the Wheatstone bridge must be carefully matched to ensure an initial balanced condition. This is conventionally done by using laser trimmed resistors. Laser trimmed resistors add greatly to the cost of the sensor, since each sensor must be placed in a test rig to permit the resistors to be trimmed by laser to the correct balance point. This is a costly process.
Aside from the disadvantages of requiring laser trimmed resistors, the conventional control circuit does not compensate well for variations in temperature, nor does it afford accurate calibration over the entire operating range.
The present invention provides a digital microcomputer-based circuit which in one embodiment can eliminate the need for expensive and time-consuming laser trimmed components. The invention also provides significantly greater accuracy and temperature fluctuation immunity than the conventional analog circuit. The invention provides a control circuit for an airflow sensor or mass airflow sensor of the type employing a flow sensing bridge having a thermal element which senses airflow as a function of electrical energy applied to the bridge. The bridge has a supply node to which electrical energy is applied and the bridge has first and second control nodes for providing an indication of the balanced and unbalanced state of the bridge.
In one embodiment, the control circuit comprises a differential amplifier for coupling to a source of electrical energy and having first and second inputs coupled to the first and second control nodes. The differential amplifier is also coupled to the supply node of the bridge circuit. The differential amplifier senses the balanced and unbalanced state of the bridge and controls the electrical energy applied to the supply node to maintain the bridge in a substantially balanced state.
The control circuit further comprises a microcomputer having a data input channel, a memory circuit and a serial communication output channel. The data input channel is coupled to the supply node of the bridge for providing the microcomputer with a first signal indicative of the electrical energy applied to the bridge. The memory circuit is preprogrammed to provide the microcomputer with instructions for converting the first signal into a second alternating signal of variable frequency at the output channel. The frequency is indicative of the measured airflow and is thus in a form for use by onboard vehicle computers.
In another embodiment of the invention, the control circuit comprises a microcomputer having first, second and third data input channels, a serial communication output channel and a second output channel. The first data input channel is coupled to the supply node of the bridge for providing the microcomputer with a first signal indicative of the electrical energy applied to the bridge. The second and third data input channels are coupled to the first and second control nodes for providing the microcomputer with an indication of the balanced and unbalanced state of the bridge.
A bridge control circuit is coupled to the second output channel of the microcomputer and also to the supply node of the bridge. The bridge control circuit has a terminal for coupling to a source of electrical energy. It controls the electrical energy applied to the supply node in response to balancing signals from the microcomputer.
The memory circuit of the microcomputer is preprogrammed to provide the microcomputer with instructions for generating the balancing signals used to maintain the bridge in a substantially balanced state and is also preprogrammed to provide the microcomputer with instructions for converting the first signal into a second alternating signal of variable frequency. The frequency is indicative of the measured airflow and is thus usable by onboard vehicle computers.
In both embodiments the microcomputer includes an analog to digital converter circuit comprising the data input channel coupled to the supply node of the bridge. The analog to digital converter has a data signal input terminal and an analog reference input terminal. The analog reference input terminal is coupled to the supply node of the bridge for providing the microcomputer with the first signal, indicative of the electrical energy applied to the bridge. The data signal input is coupled to a reference source. This causes the analog to digital converter to achieve a nonlinear response, with better than linear response accuracy at low mass airflow rates. This arrangement achieves, with an eight bit device, a ten bit or better resolution at low mass airflow rates commonly encountered in vehicle monitoring applications. The invention thus enjoys the accuracy of a ten bit analog to digital circuit at the substantially lower cost of an eight bit circuit. The cost savings at mass production volumes is considerable.
Moreover, the microcomputer can monitor ambient temperature and may be preprogrammed with temperature compensation and calibration correction tables, preferably in the form of lookup tables, to correct for any nonlinearities or inaccuracies in the bridge circuit. Ambient temperature readings can be output by the microcomputer over the variable frequency output signal, making the sensor even more versatile. Using the microcomputer to balance calibration the bridge renders a much simpler and more economical process of matching the lookup table values to the particular bridge circuit. Laser trimming is no longer required.
Another advantage of the microcomputer-based control circuit is that it provides a convenient upgrade path for changing the type of output signal being sent to the onboard vehicle computer. Present-day vehicles expect an alternating voltage of variable frequency within certain predefined ranges to indicate the mass airflow measurement. As new vehicles are designed, it is not unrealistic to expect that the type of signals may change. For example, it is anticipated that the output frequency range may need to be increased in the future in order to improve system performance. Such a change may be conveniently implemented in the present invention by simply reprogramming the memory circuits. In the alternative, it may be possible that the onboard computer of future vehicles may communicate digitally with remote sensors. In this event, the microcomputer of the invention is well suited to provide this digital communication.
The advantages of the present invention are many. For a more complete understanding of the invention and its objects and advantages, reference may be had to the following specification and to the accompanying drawings .